The invention relates to a method for determining a shift stage for a discretely shifting transmission of a motor vehicle.
From the state of the art, solutions are known wherein a transmission ratio of the motor vehicle transmission is used to adjust an optimal engine rpm. However, all these solutions require a continuously variable automatic transmission. However, the previous methods cannot be easily transferred to a discretely shifting transmission.
In discretely shifting transmissions, such as stepped automatic or automated shift transmissions, so-called pendular shifting can occur in specific operating situations wherein changes occur between the individual gears in rapid sequence. Such pendular shifting can lead to disturbances in the driving performance of the vehicle and must therefore be suppressed when possible. It is known from DE 196 25 936 to assign transmission ratio ranges to individual shift stages and in these transmission ratio ranges, shifting is suppressed. The application of the transmission ratio ranges corresponding to individual shift stages is, however, very complex. All transmission ratio ranges have to be pregiven in advance and this is especially complex when additional parameters such as a driving situation or a driver characteristic should be included.
With the method of the invention, it is possible to significantly simplify the determination of the shift stage for an optimal engine rpm without the occurrence of pendular shifting. The application of the shift stage selection has been significantly simplified in that:
a) computed and/or measured input rpms of the transmission are detected for the shift stages;
b) an rpm is fixed for each shift stage via a hysteresis circuit;
c) a difference of the input rpm of a next-higher shift stage and the rpm supplies an upshift limit and a sum of the actual input rpm and the rpm supplies a downshift limit; and,
d) a shift into a higher shift stage results when exceeding the upshift limit beyond the optimal engine rpm or a shift into a lower shift stage takes place (shift necessity) when exceeding the optimal engine rpm beyond the down-shift limit.
Furthermore, it has been shown to be advantageous to fix the hysteresis circuit based on a characteristic field in which parameters such as driver characteristic and/or an instantaneous driving situation can be included. A driver characteristic is understood to be, for example, a driving behavior which can be inputted by the driver and is with respect to consumption or power. The instantaneous driving situation can be considered especially via influences on the hysteresis circuit during city driving, stop and go traffic or for a driving operation under increased load.
In a preferred configuration of the invention, an upper limit for the input rpm is pregiven for each shift stage. When exceeding the upper limit, a shifting into a higher shift stage is forced to take place. In the same way, a lower limit rpm is pregiven for each shift stage wherein a shift into a lower shift stage takes place when there is a drop below the lower limit. With these upper and lower limits (shift stage limiters), maximum and minimum rpms for the particular shift stage can be pregiven wherein a long term stable operation is possible. The upper and lower limits are applied independently of the hysteresis circuit.
If the necessity of an upshift either by exceeding the upper limit or by exceeding the upshift limit beyond the optimal engine rpm is given, then preferably a shift into the next-following higher shift stage takes place wherein the upshift limit, which is determined for this upshift stage, has not yet exceeded the optimal engine rpm. The same can be done with the presence of a necessity for downshifting. In this case, the shift into the next-following lower shift stage takes place when the downshift limit, which was determined for this shift stage, has not yet dropped below the optimal engine rpm. In this way, the upshift or downshift can be forced continuously via sequential shift stages; however, in extreme cases, a shift over several shift stages can be made possible.
The input rpms can, in a preferred manner, be computed based on the output rpm and the given transmission ratios of the shift stages so that an additional rpm sensor at the transmission input shaft is unnecessary.